Category Archives: audio hardware

Harman / Kardon Citation eleven resources

Usual disclaimer: not my main work, we do not accept repairs. (but by golly, they do seem to find me).

In order to save anyone working on this amp a bunch of time, here is everything you need to fix this amp. The circuit is about as simple as my old sacdenhancer, so no need to go into that.

Service manual

User manual / specifications

PCB holder

A very nice construction style. Unfortunately 40 year old plastic gets brittle and I broke one just by looking at it the wrong way. So here’s a 3D print model.

EQ section hum

The equalizer is open loop and has no PSRR, so you can’t expect better than 600 uV of hum overall (spec: -80 dB relative to 6V = 600 uV).

If the EQ section picks up more than 1 mV of hum with the 60 Hz slider all the way up and maximum volume, this is due to L601 picking up stray magnetic field from the mains transformer (L601 and C607 are tuned to 60 Hz).



The citation eleven on my desk had two different L601 coils in each channel and I didn’t know which one was the original. But one was wound in such a way that it picked up over 6 mV, i.e. probably a replacement not wound symmetrically. The metal shielding is not effective on magnetic fields and in fact amplifies the field lines as it creates a short circuited winding. So the coil should be magnetically shielded and wound symmetrically.

There is a replacement coil available that is shielded correctly and brought the hum well under 300 uV. I remcomend it over the original, even.

The full DSS930 reverse engineering

Last time we looked at the amplifier output and found some strange problems. This time we isolate the DSP and verify that in fact, the problem comes from here and is actually worse than previously thought.


Now that it’s clear we need to design a replacement DSP, we need to understand all the system level communication going on. With the help of some probes a logic analyzer, a spreadsheet and educated guesswork we managed to do just that.

The circuit boards for the upgrades have been gathering dust for about a year already. Customers seem to not judge this my highest priority, and fair enough. But fear not, wheels are turning. Next up: upsampling.

Stream Unlimited CD-80 (Quad CD-99II)

No video this time, as this was supposed to be a 1-hour job… that ended up on the back burner with all the other projects for 3 months. Paying customers come first…

This is a Quad CD99-II, a friend asked me to check why it no longer read discs. Apparently it had been reading only about half of them for a while, and when that symptom was ignored, it had stopped altogether. Continue reading

Philips DSS930 – A Loudspeaker Ahead Of Its Time. Part 1: A Dark Secret

In the early 90s Philips were at the top of their game when they introduced the DSS930, a digital-input only, active DSP loudspeaker far ahead of its time. We got our hands on a pair, investigate the design and the state of these classics and discover a design flaw that has gone unnoticed for 30 years.




Functional check and repair of a Bose 1800 “Professional Solid State Dual Channel Power Amplifier”


bose1800 thermal cam

Fire in the disco


Two posts in one year? Is the world ending? Well, yes, but not just today.

I’ve been visited by a pair of handsome Bose 1800 amps over the past weeks. They needed a DC output delay / DC protection relays because apparently that tech did not yet exist in the mid 1970s. Those solid state devices were still a bit scary, I suppose. Continue reading

Setting the output voltage on a flyback switched-mode power supply

Yes, I am still alive. Sort of. 🙂

This should involve only about 3 people in the world, but hey, I should blog more even if the relevance is negligible. If this helps 1 person, I’m glad.

Not recently at all I bought a bunch of Yokogawa A1D03B supplies from Pollin (originally meant for HP printers). The idea was that they are nice to supply small / medium amplifiers for active loudspeakers. The output is 31.6V but I needed about 24V.

Continue reading

SACDenhancer revisited – part I

About four and a half thousand years ago, I designed (SA)CD player output stages with vacuum tubes and at some point came up with a transistor version, called the SACDenhancer.

Original version, ca. 2003 AD

In double blind tests this was a big preference over the built-in opamp solution. It spawned off a plethora of discrete output stages from various manufacturers and modification shops that were all the rage in the day. I had some pretty good spectrum analyzer but never could find definitive measurable differences between the standard output stage and this one.

Fast forward 15 years and I’m still getting requests for this design, so I had a PCB made. Both fabrication and measurement capabilities have moved quite a bit forward in the meantime.

Reboot, ca. 2016 AD. How nice you can now get solder mask in ‘UltrAnalog’ colour palette from China.

This version follows the original schematic but adds a second output stage for fully differential processing. As such the schematic stays ridiculously simple – just a long-tailed input stage and emitter follower output. Couldn’t be simpler (I tried. Believe me. It wasn’t good).

Basic performance

Test conditions unless otherwise specified – 1V rms differential, 997 Hz input, AES17 measurement filter, unweighted

THD+N : -99 +/- 0.5 dB

Best THD : 109 dB @ 0.7Vrms input

SNR: 100 +/- 2 dB

Frequency response linearity: +/- 0.02 dB, 20 Hz – 20 kHz

Gain linearity: +/- 0.025 dB

THD+N vs. Freq

THD vs Level


Intermodulation, 1k sine 1:1 with swept sine from 60 kHz to 6 kHz

Good news – the performance indicators are all green: no defects. This is pretty much as well-behaved as you might expect from a discrete design. But while it clearly says there’s nothing wrong with how this will sound, it does not prove it will sound better than an opamp either.

Balls to the wall

So let’s pit this thing against an opamp stage and see what happens. In the left corner, representing the heavyweight class, the Analog Devices OPA275. The challenging contestant, some ridiculous discrete design from some dude in 2003 thinking they know better. All that and more, in part 2…